1. Field of the Invention
The invention relates to a process and a device for guiding a material web and for keeping the material web spread out.
2. The Prior Art
In practical operations, various devices are known for spreading out and guiding a moving web of material. In connection, with a device known from DE-OS41 26,489, pressure elements such as belts or cords set inclined relative to the moving direction of the material web and pressing the latter against a roll entwined by such material web, produce in the material web forces, which are directed at the edges of the web and spread out the web. In addition, it is proposed to design the roll entwined by the material web as a conventional spreading roll. Guidance of the material web, thus the control of its position, is accomplished in this connection by varying the contact pressure forces of the belts in the two marginal zones of the material web by means of a force transmitter. This device suffers from the drawback that even very minor increases in the contact pressure forces trigger an over-controlling of the control system. This leads to a highly unstable control, or to a very sluggish control with adequate dimensioning of the controller. In any case, the control result achieved with such a device is unsatisfactory.
Another spreading and guiding device is known from U.S. Pat. No. 5,067,646, whose pressure elements are completely lifted off the spreading roll. When an edge sensor detects a run of the material web toward one edge, the pressure elements of the opposite marginal zone are placed on the material web, whereby such contacting takes place very gently in order to treat the material web with care. However, this two-point control has the drawback that it becomes active only when a relatively major divergence occurs. A stable and finely tuned control is not assured in this way.